Sterile filter paper discs are impregnated with crystal violet, nitrofurantoin, sodium azide, 4-nitro-o-phenylenediamine and 9-aminoacridine. Another disc, ampicillin, is commercially available. Each Salmonella tester strain, TA-100, TA-98, TA-1535, TA-1538 responds differently to each chemical. Selective use of the discs on a single plate provide a broad spectrum of reactions which are diagnostic for specific strains and can be used to confirm the genotypes of strains.